1. Field of the Invention
The present invention relates to a process for the preparation of acrylic acid by heterogeneously catalyzed gas-phase oxidation of propene with molecular oxygen over at least one multimetal oxide active material present in a reaction zone A and having the stoichiometry IMo1VbMc1Md2On  (I),where    M1 is Te and/or Sb,    M2 is at least one of the elements from the group consisting of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, B, Ce, Sn, Zn, Si and In,    b is from 0.01 to 1,    c is from >0 to 1, preferably from 0.01 to 1,    d is from >0 to 1, preferably from 0.01 to 1, and    n is a number which is determined by the valency and frequency of the elements other than oxygen in (I).
2. Description of the Background
Multimetal oxide active materials which have a stoichiometry corresponding to the formula (I) are known (cf. for example EP-A 608838, EP-A 529 853, JP-A 7-232 071, JP-A 10-57813, JP-A 2000-37623, JP-A 10-36311, WO 00/29105, Proceedings ISO'99, Rimini (Italy), Sept. 10-11, 1999, G. Centi and S. Perathoner Ed., SCI Pub. 1999, EP-A 767 164, Catalysis Today 49 (1999), pages 141-153, EP-A 962 253, Applied Catalysis A: General 194 to 195 (2000), pages 479 to 485, JP-A 11/169716, EP-A 895 809, DE-A 19835247, DE-A 10 029 338, JP-A 8-57319, JP-A 10-28862, JP-A 11-43314, JP-A 11-57479, WO 00/29106, JP-A 10-330343, JP-A 11-285637, JP-A 10-310539, JP-A 11-42434, JP-A 11-343261, JP-A 11-343262, WO-99/03825, JP-A 7-53448, JP-A 2000-51693, JP-A 11-263745 and the prior application DE-A 10046672.
In the prior art cited, the multimetal oxide active materials (I) were primarily recommended as catalysts for the heterogeneously catalyzed gas-phase oxidation and/or gas-phase ammoxidation of saturated hydrocarbons to α,β-ethylenically unsaturated carboxylic acids and/or their nitriles (e.g. propane→acrylic acid).
The prior art considered also discloses that multimetal oxide active materials having the stoichiometry (I) occur in the form of two crystalline phases which differ from one another and frequently referred to as the i-phase and the k-phase (cf. for example JP-A 11-43314, DE-A 19835247 and DE-A 10046672). Very recent investigations have led to the discovery that the X-ray diffraction pattern of both phases has the reflection of strongest intensity at the peak position 2θ=22.2±0.4°. Moreover, the X-ray diffraction pattern of the i-phase, in contrast to the k-phase, contains no reflection with the peak position 2θ=27.3±0.4°, whereas the X-ray diffraction pattern of the k-phase, in contrast to the i-phase, contains no reflection with the peak position 2θ=50.0±0.3°. Both phases additionally have a reflection with the peak position 2θ=28.2±0.4°. JP-A 11-343262 and JP-A 11-343261 recommend multimetal oxide active materials (I) which have the structure of the k-phase as catalysts for the heterogeneously catalyzed gas-phase oxidation of acrolein with molecular oxygen to acrylic acid.
JP-A 7-53448 recommends multimetal oxide active materials having the stoichiometry (I) very generally as active materials suitable for a process for the preparation of acrylic acid by heterogeneously catalyzed gas-phase oxidation of propene with molecular oxygen over catalysts present in a reaction zone. The propene to be oxidized may also contain certain proportions of propane. According to JP-A 7-53448, preferred multimetal oxide active materials (I) are those which have the structure of the k-phase. They are also used in the examples of JP-A 7-53448. However, the disadvantage of these multimetal oxide active materials is that their activity with respect to a gas-phase catalytic oxidation of propene to acrylic acid in a reaction zone is unsatisfactory. Similarly, the multimetal oxide active materials having the stoichiometry (I) of EP-A-1090684 are also not completely satisfactory for a propene oxidation.
WO 00/29105 recommends multimetal oxide active materials having the stoichiometry (I) inter alia also as catalysts for a gas-phase catalytic oxidation of C2— to C8-alkenes. WO 00/29105 also considers propane/propene mixtures as possible raw materials. According to WO 00/29105, multimetal oxide active materials having the stoichiometry (I) are however not very suitable for the abovementioned purpose when they have a crystalline structure whose X-ray diffraction pattern consists of reflections having small half-widths. Rather, according to WO 00/29105, only those multimetal oxide active materials having the stoichiometry (I) which lack short-range order possess satisfactory catalytic activity.
The general usability of multimetal oxide active materials having the stoichiometry (I) as catalysts for the heterogeneously catalyzed gas-phase oxidation of olefins with molecular oxygen is also discussed in WO 99/03825, JP-A 11-42434, JP-A 10-310539, JP-A 11-57479 and JP-A 10-28862. In none of the examples, however, is an olefin oxidation mentioned.
JP-A 8-57319 discloses that Mo and/or V-containing multimetal oxide active materials can be activated by treatment with acid.